Echo location systems are designed to identify some subset of individual reflector or target parameters. These parameters include the target position in terms like bearing and elevation angle, the range, target relative velocity and the impedance contrast which causes the echo and is also a measure of target quality. Individual targets distributed with a propagation medium make up a target field. The propagation medium can be attenuative and preferentially cause the loss of the higher frequency components of a propagating signal according to some physical law. Additionally, the propagation medium can be dispersive causing the different frequency components of a signal to travel with different velocities hence, introducing distortions of phase and equivalency of form into the propagating signal as a function of its travel.
Such known echo systems emit a signal or signals into the propagation medium; the identification process involves detecting the echo train and performing a variety of appropriate analyses. While this procedure is conceptually straight forward, there are a number of practical difficulties which act to complicate, degrade and make ambiguous such identifications.
First, there is a noise background to consider which is almost always a problem in systems where signals are transmitted and detected. Noise is defined in this instance as any contribution which is not a part of the particular identification process and has as its sources such elements as incoherent scattering by the propagation medium or even the targets themselves. There are a variety of techniques for the detection and enhancement of signals in the presence of noise.
Next, there is the inherent ambiguity between the range and the relative velocity of a target. A moving target can not only stretch or shrink a returning echo signature depending on the sense of its motion, but will also delay or speed up the time of its detection, hence affecting the range calculation. Once again, there are a variety of known techniques which can resolve this ambiguity. It is widely recognized that continuous wave signals, for example, a persistent sinusoid at a single frequency can provide good resolution of the target's relative velocity by means of the Doppler frequency shift. The companion range resolution of such a signal is necessarily poor since its character is indistinguishable from cycle to cycle. Very short duration signals are affected only slightly by target motion and while they provide good resolution in detection time, they convey little or no information about relative velocities. The chirp signal described by Klauder, Price, Darlington and Albersheim in the Bell System Technical Journal, Vol. 39, pp 745-808, July 1960, represents a compromise having ambiguity in both velocity and range. Its advantages lie rather in effectiveness of equipment utilization and the noise suppression of its companion correlation detection.
Lastly, there is the problem of resolving target angular parameters such as elevation and/or bearing. Currently, definition of angles is achieved by the use of arrays of broad-beam source or receiver elements, or else by means of narrow-beam source or receiver elements. In both cases, the space in which the target field is distributed must be scanned or viewed only one small part at a time. Scanning is accomplished either electronically by steering array beams or sequencing the operation of large numbers of elements, or even mechanically by rotating operational narrow-beam elements to new position.
The energy requirements of a scanned system are usually favorable since the entire field of potential targets need not be illuminated at once. On the negative side, however, the individual targets are then not being continuously monitored.